Support escaped backticks in template literals

This is a totally valid use but since the lexer just looks for the next
backtick as a marker for the end of the literal, things fall apart
quickly.

I added this use to the fixture, as well as one for literals without templates... just in case.
I feel like it's kind of a dumb warning to emit, but if we want to do that then fine --- will worry about cleaning up the escape handling later since it still doesn't work for ES6 unicode escapes